1. Field of the Invention
The present invention relates generally to the field of electromagnetic shielding, and more particularly, to a system and method for decoupling common mode current in data processing equipment (DPE).
2. Related Art
Common mode (CM) currents flowing along external cables, such as bus and tag cables, input/output (I/O) cables, and the like, that emerge from data processing equipment (DPE) create excessive far field electromagnetic emissions as they flow along these cables. As a condition of sale, present national and international regulations require DPE far field emissions due to CM currents to be held to relatively low levels.
A conventional method to reduce this emission mode is to return the CM currents to their source at the point of cable emergence from the equipment in question. Presently, for large DPE this method of returning the CM current is difficult. The difficulty with this conventional method is that it utilizes discrete capacitors with, for large DPE, a long meandering return path to the CM current source. An example of the conventional method to return CM current to its source is shown in FIG. 1.
FIG. 1 shows a large DPE system 100 comprising a DPE cabinet 102 for internally supporting one or more equipment housings or card cages 104. Bus and tag cables 106,108, input/output (I/O) cables 110, and the like, carry high frequency data signals, control signals, power supply lines, and the like, between the card cage 104 and the cabinet 102 via card cage connectors 112, 114 and 116 and to cabinet connectors 118, 120 and 122. From connectors 118, 120 and 122 external cables 128, 130 and 132 carry high frequency data signals, control signals, power supply lines and the like out of the cabinet 102. The conventional method returns the CM currents to their source using discrete capacitors (shown schematically at) 124 through a meandering return path 126. The meandering path 126 passes through a conductive frame (shown in cut-away view) of the cabinet 102 to the card cage 104.
The length of the meandering path 126 causes serious loss of efficiency due to low frequency (less than 30 mega Hertz (MHz)) series resonance and high inductive reactance above series resonance. The high reactance impedes the flow of CM current, thus preventing the CM current from being efficiently returned to card cage 104 and allowing it to travel onto and along external cables 128, 130 and 132.
A section of a conventional connector having a discrete decoupling capacitor is shown in FIG. 2. Within the conventional connector is a printed wire board (PWB) 202 having hole(s) 204 to receive press fit contact(s) (also called pins; not shown). A decoupling capacitor 206 is conventionally used to shunt high frequencies to ground. The decoupling capacitor 206 is electrically connected to conductive (etched copper) traces 208 to the inside of the connector housing 210 by a compliant fit with a connector housing extension 212. By electrically connecting the connector housing 210 in a conventional manner to the cabinet 102 of FIG. 1, CM currents are shunted in the conventional manner by a circuitous, meandering return path 126 back to their source in card cage 104. Because the return path is highly inductive only small CM currents can be returned to their source.
What is desired is a system and method for efficiently returning CM currents to their source so as to lower far field electromagnetic emissions.